This invention relates to a communication line interface circuit for use in a switching office and more particularly relates to a telephone line interface circuit which supplies power to a loop including subscriber apparatus from the switching office.
Generally, standard circuits for supplying power to a telephone subscriber loop are designed for a maximum length loop, e.g., a loop presenting a load of approximately 1300 ohms. For longer loops, additional circuitry will be required. For short, low impedance loops, the standard power supply circuit will tend to deliver an excessive amount of current. To protect the apparatus, the excess current is commonly shunted by means of varistors installed in the apparatus. Elimination of the high current consumption in low impedance loops will be desirable not only to protect the station apparatus but also to allow a reduction in power dissipation requirements of supply circuits and to reduce the generation of heat and the overall current consumption of the switching office. The latter two are of particular importance in small, remotely located switching offices which do not operate in a controlled environment and which may be required to run on emergency battery power for several hours in the event of commercial electrical power failure.
The desirability of a power source in a telephone exchange which provides a constant current to the loop independent of loop length has been recognized in the prior art. One prior art arrangement employs a D.C. to D.C. converter to raise the output potential in order to provide greater power for long loops and uses a standard transistor base-to-emitter feedback current regulator circuit to apply a constant current to the loop. However, in such an arrangement, considerable power is dissipated in the regulator circuit and, hence, such an arrangement is not desirable in an environment where power usage and heat dissipation must be minimized.